Heretofore, test and measurement equipment, such as logic analyzers, have employed probes having one of two kinds of probe heads. The first kind of probe head includes a simple attenuator for conveying a reduced amplitude signal from a circuit under test to the test instrument. This reduced amplitude signal is converted to digital form by a comparator in the logic analyzer. To reduce loading on the circuit under test, the second kind of probe head includes a relatively high input impedance comparator for determining the instantaneous binary state of the signal under test, and conveying a digital representation of test signal up the cable to the instrument.
Unfortunately, neither of the two probing systems described above, allow a user to see an analog representation of the signal under test. Prior logic analyzers solved this problem by including an oscilloscope plug-in module, and allowing a user to connect an oscilloscope probe to the same point at the logic analyzer probe. While this provided the desired view of the analog signal at the same test point, it also increased the loading on the circuit under test.
What is needed is a solution in which both the analog and digital signals may be acquired without causing increased loading on the circuit under test.